Plants have a powerful immune system to defend against colonization by most microbial organisms. However, virulent plant pathogens, such as Pseudomonas syringae, have developed countermeasures and inject virulence proteins into the host cell of a susceptible plant to overcome plant immunity and cause disease. Host plants include tomato plants and collard plants, such as cabbage and kale. In particular, Pseudomonas syringae pv. tomato causes an economically devastating disease called bacterial speck of tomato plants.
Bacteria control strategies are based on a combination of practices such as use of pathogen-free seed and transplants, elimination of volunteer tomato plants, resistant cultivars, and frequent application of a copper and mancozeb mixture (Jones, et al. 1986, Phytopathology 76:430-434; Jones, et al. 1991, Phytopathology 81:714-719; Sherf, et al. 1986, In: Vegetable Diseases and Their Control. John Wiley and Sons, New York; all of which are herein incorporated by reference). Chemical control has been used extensively for controlling bacterial spot. In the 1950s, streptomycin was used, but resistant bacterial strains developed and rendered antibiotics ineffective (Stall, R. E., and Thayer, P. L. 1962, Plant Dis. Rep. 46:389-392; herein incorporated by reference). However, these strategies are of limited use, especially in the tropics and subtropics where weather conditions favor infection (Kucharek, T. 1994, Plant pathology fact sheet, PP-3, University of Florida, Gainesville; herein incorporated by reference).
One method of treatment is a biopesticide product containing as active ingredients bacteriophages of Xanthomonas campestris pv. vesicatoria and Pseudomonas syringae pv. tomato “AgriPhage” EPA Registration #67986-1.
However, Pseudomonas quickly develops resistance to these treatment methods. Further, despite intensive research efforts, the molecular targets of bacterial virulence proteins important for plant disease development have remained obscure.
Therefore, there is a need for effective and economical bacterial pathogen treatments, and further, for enhancing plant immunity to virulent plant pathogens.